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Note 4 - Loans and Related Allowance for Loan and Lease Losses - Classes of the Loan Portfolio Summarized by Nonaccrual Loans and Still Accruing (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Loans, nonacrrual status	$ 8,433	$ 5,903
Loans, 90+ days past due and accruing	0	0
Commercial And Industrial [Member]
Loans, nonacrrual status	1,120	454
Loans, 90+ days past due and accruing	0	0
Real Estate Construction Porfolio Segment [Member]
Loans, nonacrrual status
Loans, 90+ days past due and accruing	0	0
Real Estate Mortgage Portfolio Segment [Member] | Residential Loan [Member]
Loans, nonacrrual status	4,002	4,034
Loans, 90+ days past due and accruing	0	0
Real Estate Mortgage Portfolio Segment [Member] | Commercial Loans [Member]
Loans, nonacrrual status	3,311	1,409
Loans, 90+ days past due and accruing	0	0
Consumer Portfolio Segment [Member]
Loans, nonacrrual status		6
Loans, 90+ days past due and accruing	$ 0	$ 0

X

- Definition

Recorded investment in financing receivables that are 90 days or more past due and still accruing.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

X

- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

+ Details

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Period Type:	instant

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=mbcn_RealEstateMortgagePortfolioSegmentMember
Namespace Prefix:
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X

- Details

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Namespace Prefix:
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X

- Details

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Namespace Prefix:
Data Type:	na
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X

- Details

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